HTML5 Boilerplate is a professional front-end template for building web applications or sites. It's chock-full of cross-browser compatibility features, CSS resets, base styles, and optional server configs to give your project the best possible start.
When you start a new project, it's unlikely you start with a blank HTML file. You probably have some kind of starting template, or your IDE builds a rough file with some elements preloaded. That's what HTML5 Boilerplate is: a starter template to use on all your projects. HTML5 Boilerplate is built and used by some of the most respected names in the industry, and is packed full of standards-compliant and browser-friendly code to make sure that all of your projects get off on the right foot.